IEEE 802.11 is a set of media access control (MAC) and physical layer (PHY) specification for implementing wireless local area network (WLAN) communication, called Wi-Fi, in the unlicensed (2.4, 3.6, 5, and 60 GHz) frequency bands. The standards and amendments provide the basis for wireless network products using the Wi-Fi frequency bands. For example, IEEE 802.11ac is a wireless networking standard in the 802.11 family providing high-throughput WLANs on the 5 GHz band. Significant wider channel bandwidths (20 MHz, 40 MHz, 80 MHz, and 160 MHz) were proposed in the IEEE 802.11ac standard. The High Efficiency WLAN study group (HEW SG) is a study group within IEEE 802.11 working group that will consider the improvement of spectrum efficiency to enhance the system throughput in high-density scenarios of wireless devices. Because of HEW SG, TGax (an IEEE task group) was formed and tasked to work on IEEE 802.11ax standard that will become a successor to IEEE 802.11ac.
In IEEE 802.11ac, a transmitter of a BSS (basis service set) of certain bandwidth is allowed to transmit radio signals onto the shared wireless medium depending on clear channel assessment (CCA) sensing and a deferral or backoff procedure for channel access contention. For a BSS of certain bandwidth, a valid transmission sub-channel shall have bandwidth, allowable in the IEEE 802.11ac, equal to or smaller than the full bandwidth of the BSS and contains the designated primary sub-channel of the BSS. Based on the CCA sensing in the valid transmission bandwidths, the transmitter is allowed to transmit in any of the valid transmission sub-channels as long as the CCA indicates the sub-channel is idle. This dynamic transmission bandwidth scheme allows system bandwidth resource to be efficiently utilized.
An enhanced distributed channel access protocol (EDCA) is used in IEEE 802.11ac as a channel contention procedure for wireless devices to gain access to the shared wireless medium, e.g., to obtain a transmitting opportunity (TXOP) for transmitting radio signals onto the shared wireless medium. The simple CSMA/CA with random back-off contention scheme and low cost ad hoc deployment in unlicensed spectrum have contributed rapid adoption of Wi-Fi systems. Typically, the EDCA TXOP is based solely on activity of the primary channel, while the transmit channel width determination is based on the secondary channel CCA during an interval (PIFS) immediately preceding the start of the TXOP. The basic assumption of EDCA is that a packet collision can occur if a device transmits signal under the channel busy condition when the received signal level is higher than CCA level.
Today, Wi-Fi devices are over-populated. Dense deployment has led to significant issues such as interference, congestion, and low throughput. Raising CCA levels has been shown to increase spatial re-use, which leads to significant increase in the network throughput in some dense deployment scenarios. In dense deployment scenario with multiple small BSS footprints in which APs and non-AP STAs are mostly exchanging frames at the highest MCS (modulation and coding), the baseline CCA level −82 dBm leads to excessive deferral and thus lower overall throughput. By increasing CCA level (OBSS interference) for all BSSs in the scenario, the operating SNR is still above the level required for max MCS. The specific link throughput does not degrade, but CCA deferral is reduced (likelihood of channel access increased) leading to increased network throughput.
In general, increasing CCA levels for inter-BSS packets can enhance the spatial reuse because more simultaneous transmissions can happen in multiple OBSSs. However, increasing CCA levels for inter-BSS packets may cause many stations that only support lower MCSs contends and win the channel access. As a result, network throughput is not optimized. This problem can be more severe if dynamic CCA rules are applied. To gain more opportunities to win channel access contention, every station is inclined to use the highest CCA level by reducing MCS such that overall network throughput can decrease dramatically. In worst scenarios, most packets are modulated with MCSO.
Consider a WLAN based on IEEE 802.11 standards. The CCA rules in IEEE 802.11a/g/n/ac only determine if a STA can contend for channel access or not. All STAB whose received signal strengths are below the CCA level have the equal probability to win the channel access through EDCA/CCA contention. However, in a dense WLAN, STAB at different locations very likely experience different interferences. If STA1 experiences more interference than STA2, then it is desirable that STA2 wins the channel access contention if both STA1 and STA2 have data to transmit. This is because STA2 is allowed to transmit with higher power than STA1 such that both introduce the similar amount of interference to OBSSs. STA2 is able to transmit with higher MCS than STA1 such that the network throughput is enhanced. Therefore, STA2 has larger spatial reuse capability, which can be transferred to higher MCS or less interference to OBSSs or power save.
It is thus desirable to have a channel access scheme with a channel contention that is favorable to stations with large spatial reuse capabilities.